The present invention relates to an electrophotographic toner, and more particularly, relates to an electrophotographic toner which is used for image forming apparatuses such as electrostatic copying machine and laser beam printers.
The electrophotographic toner is normally produced by dispersing a colorant, an electric charge controlling material, etc. in a suitable fixing resin, followed by pulverizing and screening.
As the above electric charge controlling material, various compounds such as dyes, etc. have been used according to the charging polarity of the toner. As the positively charging toner, the use of a quaternary ammonium salt, which contains no metal and is white, has recently been studied in view of consideration for environments, correspondence to a color toner, etc.
However, since the crystallizability of a normal low-molecular weight quaternary ammonium salt is high and the dispersion properties to the fixing resin are not sufficient, the charging properties are inferior. Therefore, the charged amount becomes lower than a predetermined value and it charged to a reverse polarity, thereby causing image failures, such as the so-called image fog wherein toner is adhered to blanks in an formed image, and toner scattering, etc. There is also a problem of the charging stability, that is, scattering of the image density arises when the image is formed repeatedly.
It can also be considered that the amount of the low-molecular weight quaternary ammonium salt to be added is increased in order to obtain a sufficient charged amount. However, since the quaternary ammonium salt does not have sufficient dispersion properties to the fixing resin as described above, the strength of the toner is decreased when a large amount of it is added. In addition, the toner is liable to be broken when it is repeatedly mixed with stirring in a developing apparatus of the image forming apparatus, thereby causing the problem that the powder of the fixing resin and quaternary ammonium salt generated from the broken toner contaminate the interior of the developing apparatus, or they adhere to a magnetic carrier constituting a two-component developer, together with the toner, to cause a so-called spent contamination.
Furthermore, there is also the problem that it is not easy to adjust or confirm the dispersion properties of the low-molecular weight quaternary ammonium salt to the fixing resin, thereby making it difficult to control the quality of the toner.
In Japanese Unexamined Patent Publication Nos. 60-067961, 60-073547 and 60-217370, there is disclosed that a resin (electric charge controlling resin) wherein a group corresponding to the quaternary ammonium salt (hereinafter referred to as a "quaternary ammonium salt group") is introduced at the side chain of a polymer which is compatible with a fixing resin is used as an electric charge controlling material.
Such an electric charge controlling material is compatible with the fixing resin uniformly on the whole and, therefore, it has the advantage that high charging properties can be imparted to the toner by adding a small amount of it in comparison with the quaternary ammonium salt.
However, the charging properties of the toner are insufficient when the quaternary ammonium salt is merely subjected to polymer-processing. Therefore, the same problem as that in case of using the low-molecular weight quaternary ammonium salt arises, or other characteristics of the toner, e.g. fixing properties, offset resistance, the strength of the toner, etc. are likely to be deteriorated.